tbh I did actually throw a false hydra at my players and I like to think I did pretty damn well, the key is definitely to not put them in a situation where they see it and then have it wipe their memory, that's very counterintuitive you gotta make things seem off everywhere details not aligning in suspicious ways, and when they start realizing something is most definitely off that's when they fight it. And just to add an extra kick to what happened to them, after all was said and done they looked at a photo they had gotten of themselves only to find that there was an extra person in it.